Jupiter's intricate, swirling ring system is formed by dust
kicked up as interplanetary meteoroids smash into the giant planet's
four small inner moons, according to scientists studying data from
NASA's Galileo spacecraft. Images sent by Galileo also reveal that
the outermost ring is actually two rings, one embedded within the
other.

The findings were announced today by scientists from Cornell
University, Ithaca, NY, and the National Optical Astronomy
Observatories (NOAO), Tucson, AZ, at a news briefing held at Cornell.

"We now know the source of Jupiter's ring system and how it
works," said Cornell astronomer Dr. Joseph Burns, who reported on the
first detailed analysis of a planet's ring system, along with Maureen
Ockert-Bell and Dr. Joseph Veverka of Cornell, and Dr. Michael Belton
of NOAO.

"Rings are important dynamical laboratories to look at the
processes that probably went on billions of years ago when the Solar
System was forming from a flattened disk of dust and gas," Burns
explained. Furthermore, similar faint rings probably are associated
with many small moons of the Solar System's other giant planets. "I
expect we will see similar processes at Saturn and the other giant
planets," Burns said.

In the late 1970s, NASA's two Voyager spacecraft first revealed
the structure of Jupiter's rings: a flattened main ring and an
inner, cloud-like ring, called the halo, both composed of small, dark
particles. One Voyager image seemed to indicate a third, faint outer
ring.

New Galileo data reveal that this third ring, known as the
gossamer ring because of its transparency, consists of two rings.
One is embedded within the other, and both are composed of
microscopic debris from two small moons, Amalthea and Thebe.

"For the first time we can see the gossamer-bound dust coming
off Amalthea and Thebe, and we now believe it is likely that the main
ring comes from Adrastea and Metis," Burns said. "The structure of
the gossamer rings was totally unexpected," Belton added. "These
images provide one of the most significant discoveries of the entire
Galileo imaging experiment."

Galileo took three dozen images of the rings and small moons
during three orbits of Jupiter in 1996 and 1997. The four moons
display "bizarre surfaces of undetermined composition that appear
very dark, red and heavily cratered from meteoroid impacts," Veverka
said. The rings contain very tiny particles resembling dark, reddish
soot. Unlike Saturn's rings, there are no signs of ice in Jupiter's
rings.

Scientists believe that dust is kicked off the small moons when
they are struck by interplanetary meteoroids, or fragments of comets
and asteroids, at speeds greatly magnified by Jupiter's huge
gravitational field, like the cloud of chalk dust that rises when two
erasers are banged together. The small moons are particularly
vulnerable targets because of their relative closeness to the giant
planet.

"In these impacts, the meteoroid is going so fast it buries
itself deep in the moon, then vaporizes and explodes, causing debris
to be thrown off at such high velocity that it escapes the
satellite's gravitational field," Burns said. If the moon is too
big, dust particles will not have enough velocity to escape the
moon's gravitational field. With a diameter of just eight kilometers
(five miles) and an orbit that lies just at the periphery of
the main ring, tiny Adrastea is "most perfectly suited for the job."

As dust particles are blasted off the moons, they enter orbits
much like those of their source satellites, both in their size and in
their slight tilt relative to Jupiter's equatorial plane. A tilted
orbit wobbles around a planet's equator, much like a hula hoop
twirling around a person's waist. This close to Jupiter, orbits
wobble back and forth in only a few months.

Jupiter's diameter is approximately 143,000 kilometers (86,000
miles). The ring system begins about 92,000 kilometers (55,000
miles) from Jupiter's center and extends to about 250,000 kilometers
(150,000 miles) from the planet.